The practice of clinical pathology centers around the microscopic analysis of biopsies obtained from the body. Although tissue biopsies are fundamentally three-dimensional, they must be sectioned for two-dimensional analysis by light microscopy because of the opaqueness of most biological specimens. As a consequence, multiple two-dimensional samples must be prepared for each biopsy. Sample preparation can be very costly for each section. Moreover, to accurately characterize the properties of the entire 3-D sample, a large number of sections must be prepared.
Acoustic microscopy is a well established technique dating to the early 1970s. The most recognized system was produced in the Applied Physics Department at Stanford University by Calvin Quate. U.S. Pat. Nos. 4,006,444; 4,028,933; 4,267,732; 4,430,897; and 5,319,977 disclose various acoustic microscopes wherein Mr. Quate is a named inventor.
Several small commercial versions of this microscope, and similar microscopes, have been produced over the last decade. All of these microscopes are inherently two-dimensional, where an image is commonly obtained through some form of mechanical scanning.
Short pulse laser excitation of acoustic waves is also a well established technique for ultrasonic frequencies less than 100 MHz. A large body of work was done on this at IBM by von Gutfeld in the early 1980s as described in the U.S. Pat. No. 4,512,197 to von Gutfeld et al.
Recent work by a group in the Physics Department at Brown University led by Tauc and Maris has shown that laser excitation can be extended to produce ultrasonic pulses at frequencies greater than 1 GHz. U.S. Pat. No. 4,710,030 in the name of Tauc et al. discloses some of this work.
Synthetic Aperture techniques are common in ultrasonic and RADAR systems as disclosed in the U.S. Pat. Nos. 5,269,309 and 5,465,722 to Fort et al. For example, Synthetic Aperture Radar (SAR), pioneered by ERIM over two decades ago, is now routinely used in many forms of surveillance.
However, all work to date on laser-generated, high frequency, acoustic waves uses weakly focused optical sources, resulting in spatially extended excitation (i.e., equivalent aperture many ultrasonic wavelengths across). Such excitation produces nearly plane wave propagation of the resultant ultrasonic pulse.